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THE
80286
INSTRUCTION SET
LOCK-Assert
BUS
LOCK
Signal
Opcode
Instruction Clocks
Description
FO
LOCK o
Assert BUSLOCK signal
for
the next instruction
FLAGS MODIFIED
None
FLAGS UNDEFINED
None
OPERATION
LOCK
is
a prefix that
will
cause the BUS LOCK signal of the 80286 to
be
asserted
for
the duration
of the instruction that it prefixes. In a multiprocessor environment, this signal should
be
used
to
ensure
that the
80286 has exclusive use of any shared memory while BUS LOCK
is
asserted. The read-
modify-write sequence typically used
to
implement TEST-AND-SET
in
the 80286
is
the XCHG
instruction.
The
80286 LOCK prefix activates the lock signal for the following instructions: MOYS, INS, and
OUTS. XCHG always asserts BUS LOCK regardless of the presence or absence of the LOCK prefix.
PROTECTED MODE EXCEPTIONS
#GP(O)
if the current privilege level
is
bigger (less privileged) than the .I/O privilege level.
Other exceptions may
be
generated by the subsequent (locked) instruction.
REAL ADDRESS MODE EXCEPTIONS
None. Exceptions may still be generated
by
the subsequent (locked) instruction.
B-68